Viking fortresses are pretty badass by nature, but this latest discovery is still special.

Last week, archaeologists announced the excavation of a stone fortress — Vallø Borgring — on the Danish island of Zealand. Presumed to date to the 10th century and the reign of King Harald Bluetooth, the fortress is distinguished by its circular shape — a style unique to Denmark, Lasse Sonne, a Viking historian from the University of Copenhagen, told the Telegraph. It's only the fifth circular fortress ever found, the third-largest of its kind and the first to be unearthed in over 60 years.

Here are some other cool facts about Vallø Borgring:

1. It took a lot of work to find it.

Source: Museum Sydøstdanmark

"The discovery has been a piece of detective work," archaeologist Søren Sindbæk told Past Horizons.

The archaeologists figured one fortress was undiscovered on Zealand, so they looked to the landscape, he explained. The new find is situated where the old main roads met and reached into the Køge river valley, which in the Viking Age was a navigable fjord and natural harbor. "From there we worked our way forward step by step."

2. There was a lot of cool technology involved.

Source: Museum Sydøstdanmark

Once the site was identified, the archaeologists used cutting-edge techniques to conduct a noninvasive survey, which successfully gave them a sense of the layout.

By measuring variations in the earth's magnetism, they were able to construct a "ghost image" of the fortress in a few days. "From this survey we knew exactly where we had to put in excavation trenches to get as much information as possible about the mysterious fortress," Sindbæk said.

The fortress, which measures 145 meters from side to side, has a design that mirrors that Viking ring fortresses known as Trelleborgs, Danish Castle Centre curator Nanna Holm told Past Horizons. "Our investigations show that the new fortress was perfectly circular and had sturdy timber along the front. ... It is a marvelous find."

4.It shows a new side of Viking culture.

Source: Museum Sydøstdanmark

Not only were the Vikings great fighters, but they were also skilled artists and architects.

"The Vikings have a reputation as berserkers and pirates. It comes as a surprise to many that they were also capable of building magnificent fortresses," Sindbæk said. "The discovery of the new Viking fortress is a unique opportunity to gain new knowledge about Viking war and conflicts, and we get a new chance to examine the Vikings' most famous monuments."

5. But it was still a military installation — and a lot of violent fighting probably took place here.

Source: Museum Sydøstdanmark

"We can see that the gates were burned down; in the north gate we found massive, charred oak posts," Holm told Past Horizons, adding that these traces can be used to figure out what conflicts played out here and how old the fortress is. "If we can establish exactly when the fortress was built, we may be able to understand the historic events of which the fortress was part."

6. This is good news for future excavations.

Source: Museum Sydøstdanmark

While it's probable that the fortress was constructed during the time of King Harald Bluetooth, much excavation needs to be carried out to confirm the theory. In the meantime, this news serves as further proof that there is still much left to discover — in Denmark and beyond.

"This is really exciting. The excavation has confirmed far more than we dared hope, but there is much more to learn," Sindbæk said. "The next big question is whether there were large buildings inside the castle, as there are in the known Trelleborg fortresses. The find also raises the question as to whether there will be more new Viking fortresses to discover. The exploration will be a wonderful journey of discovery."